Particles differing in density may be separated by using hydrostatic forces, such as used in separating coal from rock and pyrites in aqueous and non-aqueous slurries, but that entails large expenditures of energy for removing the water, drying the product, and treating the waste water. The usual methods for separating particles of differing density utilize hydrostatic forces as in coal washing or ore beneficiation. Those operations commonly employ a water slurry that flows slowly through a system of troughs. The denser particles drift toward the bottom, leaving the less dense particles in the upper layers. The flow is usually laminar, and a large number of stages may be employed a where the ore concentration in the initial feed may be as low as 0.05%,.as is the case in copper mining operations. In coal cleaning, on the other hand, the bulk of the raw feed material is coal, while the rock and/or pyrite to be removed is only 1 to 20%. Major disadvantages of this wet slurry approach are the contamination of the water required and the large amount of energy required to dry the product where it constitutes the bulk of the feed, as is the case for coal cleaning operations. If gas-fluidized beds are employed to avoid those problems, the pumping power required is substantial and the dust problems are formidable.
Both of those problems may be alleviated somewhat by employing an “air flow jig” in which a thin bed of crushed coal flows over a vibrated plate that is perforated to introduce a stream of air to fluidize the bed. However, this system is not sufficiently attractive to have led to more than a very limited commercial use. Air flotation may be employed, but this is effective only for mixtures having a small range of particle size and presents difficult problems with dust.
A low attrition dry process is particularly advantageous for removing mineral matter from low-rank coals, such as lignite, in a which the lumps are quite frangible even when dry, and their frangibility is greater when wet. Many efforts to effect density separation through the use of vibrating beds have been made in the past, but none has been effective. Rather than acting as separators, vibrating beds have proved to be an excellent means for obtaining homogeneous mixtures of granular materials differing in density, e.g., corn starch and foundry sand, for making dry sand cores. All of these efforts have employed vibratory motions given by the vast majority of the vibrating machines on the market that produce a simple linear vibratory motion, or the balance that produce a simple whirl.
At first thought, one would expect that vibration-fluidized beds could be used to give a dry process. On the surface it would seem that vibration would aid the usual separation operation, and that when a bed is fluidized by vibration the dense particles should sink to the bottom. One patent that has been granted for such a separation process is U.S. Pat. No. 4,894,148 (the '148 patent), issued Jan. 16, 1990. The abstract of the '148 patent states that “[t]he mass is received in a trough-shaped [container] and subjected to vibration causing high density constituents to segregate downwards and low density constituents to segregate upwards, so that the fractions may be removed at different layer levels.” FIG. 1 of the '148 patent shows an old-fashioned bathtub-shaped casing with three outlets, one at the top, one at an intermediate level, and one at the bottom. However, no experimental evidence of its effectiveness is cited in the '148 patent, and people with extensive experience with vibrating beds (e.g., Prof. Arthur M. Squires at VPI&SU) state that they have looked for but never found any evidence of particle separation as a consequence of differences in particle density in conventional vibrating beds.
Several patents have been issued for the use of vibrating beds to mix granular materials differing in particle size, density, and character. U.S. Pat. No. 4,493,556, issued Jan. 15, 1985, cites experimental confirmation of the outstanding-effectiveness of the linear motion vibrating bed for mixing flour and sand (apparently for making dry sand cores for foundry molds). The effectiveness of the linear motion vibrating bed for mixing materials of different character, particle size, and density negates the use of simple linear vibratory motion for separating particles on the basis of density with a simple linear vibratory-motion. Another significant point is that there is a tendency for larger particles to work toward the surface when operating with granular materials differing in particle size but having the same grain density. The effect is small, but nonetheless it definitely is present.
All present methods and apparatus have employed the usual simple linear vibratory motion, either vertically or inclined to the vertical at some angle. A need exists for the effective separation of granular matters of different densities that overcomes the aforementioned problems.